Sildenafil (English: Sildenafil), also known as Xiduofen, is a drug for treating male erectile dysfunction that was accidentally invented during the development of drugs for treating cardiovascular diseases. It is a prescription drug under legal regulations and can only be purchased and taken after a doctor's diagnosis and prescription.

It is generally known by its commercial name Viagra (registered name in mainland China, Viagra in Taiwan, Hong Kong and Macao), commonly known as "Viagra" in mainland China and Hong Kong, and "the little blue pill" in Taiwan. Currently, Pfizer's patent on sildenafil has expired, and other manufacturers can legally manufacture the same drug.

Sildenafil is a type V phosphodiesterase inhibitor that acts on type 5 phosphodiesterase (PDE5). Phosphodiesterase is a negative regulator of the NO-cGMP pathway. It inhibits PDE5 from breaking down cGMP and increases the amount of protein kinase G (PKG) activated, thereby dilating blood vessels and lowering blood pressure, which can treat pulmonary hypertension and altitude sickness.

Sildenafil can highly selectively inhibit the activity of PDE5 in the human body. PDE5 is expressed at a very high level in the corpus cavernosum of the penis, but is expressed at a lower level in other tissues and organs of the human body. After taking sildenafil, the vascular smooth muscle of the corpus cavernosum of the penis relaxes under the action of the drug, blood flow increases, the corpus cavernosum becomes engorged, and the penis becomes erect, thereby producing a therapeutic effect on erectile dysfunction.

The average duration of action for patients with erectile dysfunction after oral administration is 27 minutes (clinical data ranges from 12 to 70 minutes)

The main adverse reactions of sildenafil come from its inhibition of phosphodiesterase in other parts of the human body.

Common adverse reactions to sildenafil include dizziness, nasal congestion, indigestion, and transient visual abnormalities, which may manifest as abnormal blue/green color discrimination, increased light sensitivity, or blurred vision. Most of these adverse reactions are caused by the associated smooth muscle relaxation. [7]

Other more serious adverse reactions of sildenafil include: decreased blood pressure and decreased cardiac output when lying down. In addition, clinical studies have shown that sexual intercourse after taking sildenafil increases the risk of heart abnormalities, including symptoms such as angina pectoris, dizziness, nausea, and may cause sudden cardiac death.

In addition, there have been occasional reports of abnormal erections after taking sildenafil.

It is not recommended for people with impaired liver function, severe hypertension (above 170/100 mmHg), severe hypotension (below 90/50 mmHg), a history of stroke, myocardial infarction or cardiovascular risk factors (cardiovascular disease, heart failure, angina pectoris), and hereditary degenerative retinal hemorrhagic diseases to use Viagra.
It is also not recommended for patients with nonarteritic anterior ischemic optic neuropathy.

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